docs: def edge_history(self, source, target, attribute):
          """
          Returns a dictionary of attribute vales for each Graph in the
          :class:`.GraphCollection` for a single edge.
  
          Parameters
          ----------
          source : str
              Identifier for source node.
          target : str
              Identifier for target node.
          attribute : str
              The attribute of interest; e.g. 'betweenness_centrality'
  
          Returns
          -------
          history : dict
          """
  
          return {attr['graph']: attr[attribute] for i, attr
                  in self.master_graph.edge[source][target].items()}